Yes De-aerators can corrode ...

Yes De-aerators can corrode because by their nature oxygen will be present.  How you treat them depends entirely what the boilers are they are supplying.  If they are relatively low pressure up to 25 bar, then I would dose caustic to the de-aerator.  Oxygen scavengers are normally dosed after the de-aerator.  As others have said, if chlorides are high then corrosion can occur because localised elevated chlorides can take place.  Higher pressure or water tube boilers will need to be looked at differently.